The devices shown in FIGS. 30 and 31 are conventional IC chip mounting structures for which lead frame element 1 shown in FIG. 32 can be used. The entire body of lead frame 1 can be formed of a metal (for example, alloy 42, materials to which an iron/nickel alloy plating is applied after forming, copper alloys, copper, or the like), and a lead frame section 8 can be made of a lead frame external framework 2 and a large number of internal leads 7 extending radially to locations on the interior sides of the lead frame, integrally formed by etching, stamping, or the like.
In the lead frame section 8, each of outer leads 6 were connected by connecting sections 9 (dam bars) of a wire form at each of its sides, and these connecting sections were integrally formed with the external framework 2 at each corner of the lead frame section 8.
An IC chip 10 is mounted on a rectangular mounting pad 11 (die pad) of larger surface area than the chip. Support pins 12 which support the mounting pad connect to the mounting pad 11, and are integrally formed with the external framework 2 at the corners. FIG. 4 shows a slot used for the lead frame element positioning.
Using the lead frame element 1 made in this manner, IC chip 10, shown in FIGS. 30 and 31, is affixed on top of mounting pad 11 with a silver paste 14, pad 17 is bonded to inner lead 7 by wire 15, and then the entire body sealed with a resin 16 such as an epoxy resin. In FIG. 31, a single dot segmented line shows the outer line of the resin of the sealing resin 16, and a double dot segmented line shows the position of the IC chip 10.
In the manufacture of this package 13, first, as shown in an enlarged view in FIG. 33, the IC chip 10 is mounted on top of mounting pad 11 by silver paste 14. Then, as shown in FIG. 34, during wire bonding, a heating element 18 (heater insert or heating block) is made to contact the mounting pad 11 from below, and simultaneously with supporting the mounting pad 11, heat from the heater propagates to the bonding section (pad) 17.
The wire bonding is conducted by attaching a wire 15 by a capillary 19 to a bonding pad 17 while applying heat and ultrasonic energy, and then directing inner leads 7 as shown by the arrow 20 and pressure bonding.
In this prior art package construction mounting pad 11 for the IC chip 10 is larger than IC chip 10, mainly for the following reasons. First, at the time of the bonding, assuming that the mounting pad 11 was made smaller than the IC chip 10, the IC chip 10 was supported by the mounting pad 11 only in the center section, and because the edges of the chip were unsupported, the IC chip 10 tipped during the application of ultrasound due to capillary 19, it became unstable, the ultrasonic energy could not be used efficiently, and, because the heat from the heater insert 18 was not transmitted to the IC chip 10 efficiently, the bonding was incomplete. Thus, making the mounting pad 11 smaller than the IC chip 10 was ill-considered by persons in the industry.
At the time of positioning the resin-sealed package 13 in the desired position in relation to the circuit pattern on top of the printed circuit board and fixing by soldering by solder reflow or the like, cracks appeared in the resin 16 due to thermal shock.
Until now, if the total thickness of the package 13 was sufficient, cracks did not readily appear, but with the recent change to a thinner package and a larger chip, and with the small mounting pad 11, the boundary surface of the resin 16 easily peels and cracks entering from the edge of the pad 11 are frequently observed.
This is because if the ambient moisture where the package is stored penetrates into the resin, and that moisture is present in the spaces between the lead frame and the resin during the solder reflow, the moisture changes to steam, abruptly expands, and a so-called steam explosion occurs, where the resin peels from the lead frame.
Due to the differences in the thermal expansion coefficients of the mounting pad, IC chip, and the resin, distortion remains in the package after the resin molding and completion of the curing, warpage is generated, and there are cases in which the package itself becomes completely deformed.
Another cause of the cracks can be suggested: because the silver paste 14 used to adhere the IC chip 10 is extremely moist, it may bring about a steam explosion during heating, such as during solder reflow.
Due to the same type of phenomenon as was mentioned above, and due to the moisture that has collected in the area of the silver paste 14, bringing about a steam explosion, pad 11 deforms, the resin 16 is stressed, and cracks appear, especially from the edge of the pad 11.
Therefore, the inventors of the present invention, in order to solve the defects in the package construction of the prior art, have already proposed a mounting pad construction that is smaller than the IC chip by the Japanese Patent Application No. Hei 51993!-165248 (hereinafter referred to as the invention of the previous application). This mounting pad construction is like that shown, for example, in FIG. 35, and according to this mounting pad construction, remarkable effects can be obtained such as (1) to (5) below.
(1) Because the mounting pad 27 is small, the trapped moisture that has penetrated into the vicinity of the pad 27 is reduced by the corresponding amount, the distortion due to the differences in the thermal expansion and contraction between the pad 27 and the resin becomes smaller, there is no peeling between the pad 27 and the resin 16 due to the steam explosions as was described above during heating (for example, during solder reflow), and the appearance of cracks in the resin is eliminated, or the cracks are significantly reduced.
(2) In this case, due to the fact that in general the adhesion between the silicon (IC chip 10) and the epoxy resin (sealing resin 16) is extremely good, since the resin 16 directly adheres to the IC chip 10 in the periphery of the pad 27, it is difficult for peeling to occur between the interface of the resin 16 and the IC chip 10, and the possibility that the moisture will collect in this boundary and cause a steam explosion is reduced one order of magnitude.
(3) Moreover, since the silver paste 14 fills the depression 27a that is provided in the pad 31, and does not protrude out from the joint face of the IC chip 10 and pad 27, and because the amount of silver paste that is used can be reduced, in addition to reducing the cost, the release and diffusion of moisture, and furthermore, the release of absorbed gases which originate in the silver paste becomes slight, and cracks are not generated in the resin 16 due to steam explosions, or the cracks can be significantly reduced. One can get by without using an epoxy resin having a low stress for the resin 16, and thus, the amount of gas released from the silver paste can be further reduced.
(4) Because the silver paste 14 fills in the depression 27a, the total thickness of the mounting pad 27, the silver paste 14, and the IC chip 10 becomes thinner just by the thickness of the silver paste (for example, about 30 .mu.m) compared to the prior example of FIG. 30, and the total thickness can be reduced when forming the package.
(5) Because the mounting pad 27 is smaller than the IC chip 10, the size of the chip does not depend on the size of the mounting pad 27, and a greater number of chip sizes can be mounted.
Because the manufacturing method for a package based on the present invention of the previous application uses a heater that is provided in the insertion cavity of the mounting pad 27, and the bonding can be conducted by directly supporting the IC chip 10 on its periphery, it exhibits the remarkable operating effects described in (6) and (7) below.
(6) At the time of wire bonding, even though the mounting pad 27 is small, this can be positively supported on top of the heater; moreover, the support is more stable because the pad 27 is inserted inside the concave area of the heater. Therefore, the wire bonding can be stably accomplished.
(7) Since the IC chip 10 is made to directly contact the heater, the heat transmission from the heater to the IC chip (in particular, pad 17) is excellent and the ultrasonic energy is not lost, the bonding strength can be excellently maintained, and even if the ultrasonic energy of the capillary 19 and the heat from the heater are lowered, it is still possible to conduct sufficient bonding.
However, in the case in which the pad is smaller than the IC chip but larger than the width of the support pin 28 (hereinafter called a small pad), in contrast to the advantages just discussed, there are the disadvantages described below, and the inventors of the present invention found that there is more room for improvement in the invention of the previous application.
As for the misalignment of the mounting pad 27 (die pad), as shown in FIG. 36, depending on the thickness of the IC chip 10 that is arranged on top of it and the thickness of the mounting pad 27, a flat step difference D of 0.17 to 0.22 mm is formed between the inner lead 7 and the mounting pad 27 and the support pin 28. As shown in FIG. 36, it is set between the bottom mold 25a and the top mold 25b of the package resin molding metal mold, and an epoxy type resin 16 is injected from the ejection nozzle 26 and it is resin sealed. At this time, the horizontal step difference D is provided so that the ejected resin 16 flows uniformly in the space in the upper and lower cavities within the mold, and the IC chip 10 is made to occupy a central position.
In order for the semiconductor device to be properly sealed by the resin 16 being injected uniformly in the upper and lower mold cavities by centering the IC chip 10, it is very important that the semiconductor device to be resin-sealed is first supported and held at the proper position.
However, as shown in FIG. 35, because the diameter of the support pins 28 of the small pad 27 decreases, the length increases by exactly the amount that the diameter of the pad 27 is reduced, so their bending strength is reduced. Therefore, as shown in FIG. 34, during wire bonding, due to the tension of the bonding wire 15, the support pins 28 are pulled by the bonding wire 15, as shown in FIG. 37, and are bent in the direction of the arrow B.
Thus, as shown in FIG. 37, if the resin-sealing is conducted with the IC chip 10 lifted up, because the volume of the cavity of the bottom mold increases, the resin 16 quickly flows into the cavity of the bottom mold. Therefore, due to the pressure of the resin 16 that has flowed in, the IC chip 10 is pushed up still more, and as a result, it is resin-sealed in the state shown in FIG. 38.
The purpose of the present invention is to offer a semiconductor device, its manufacturing method, and lead frame, where the bending of the support pin in the manufacturing processes, such as at the time of wire bonding and at the time of resin sealing, can be significantly reduced or prevented.